需求：

 输入银行卡号需要四个数字一组，想找个现成没找到，没办法只有自己写了

构思:

需要监听输入框的变化，不断修改输入框的输入文本的格式

实现:

第一步：
在分类中添加格式化字符串代码和移除空格代码

extension String{
   /*
    *去掉所有空格
    */
    var removeAllSapce: String {
        return self.replacingOccurrences(of: " ", with: "", options: .literal, range: nil)
    }

   /// 分组格式化
    /// - Parameter joined: 间隔字符串
    /// - Returns: 格式化好的字符串
    public func formateForBankCard(joined: String = " ") -> String {
           guard self.count > 0 else {
               return self
           }
           let length: Int = self.count
           let count: Int = length / 4
           var data: [String] = []
           for i in 0..<count {
               let start: Int = 4 * i
               let end: Int = 4 * (i + 1)
               data.append(self[start..<end])
           }
           if length % 4 > 0 {
               data.append(self[4 * count..<length])
           }
           let result = data.joined(separator: " ")
           return result
       }

}

使用方式

class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var textView: UITextField!
    override func viewDidLoad() {
        super.viewDidLoad()

        self.textView.delegate = self
        self.textView.addTarget(self, action: #selector(textChange), for: .editingChanged)
    }

    @objc func textChange(){
        let text = self.textView.text
        let tempText = text!.removeAllSapce
        self.textView.text = tempText.formateForBankCard()
    }
}